Mine measure about .108 thick with paint. That's probably 12 gauge.
The inside dimension is the important one - it should be pretty snug on the spring eye. You don't want the spring eye to float on the bolt, but pinch it between the side of the box. I measure 3.150, but if I were you, I would bolt the pieces to the spring, then tack weld it. Remove the spring and tack a brace across the bottom edges to keep the sides from pulling in.
By the way, they'e roughly 5" tall x 11" long. Leave them big and trim them after welding in place.
The instructions are a complete joke. Looks like a kid make them with crayons. The article here is much, much better than what MP gives you. I aligned mine with the stock hangers by using two pieces of 5/8 threaded rod and a coupler. The rod runs all the way through both stock hangers, and the boxes. Leave it in place while you weld the boxes in.
Mark